The FBI and Homeland Security Department sent two bulletins Monday to police around the country but said they know of no specific plots and did not mention the ongoing high-profile investigation of a possible terrorist plot involving a Colorado man and associates in New York City.

In the bulletins — obtained by The Associated Press — officials urged law enforcement and private companies to be vigilant. The warnings followed on the heels of a similar warning about the vulnerabilities of mass transit systems.

The bulletin on stadiums notes that an al-Qaeda training manual specifically lists "blasting and destroying the places of amusement, immorality, and sin... and attacking vital economic centers" as desired targets of the organization.

“It is our role in law enforcement to constantly maintain a high level of vigilance," Washington, D.C., Police Chief Cathy Lanier said in a statement released Tuesday. "We do this through our partnerships with the local and federal law enforcement communities and the private sector. While these bulletins do not indicate that there is a specific threat, they do remind us that we always need to remain on our guard.”

"While DHS and FBI have no information regarding the timing, location or target of any planned attack, we believe it is prudent to remind transit authorities to remain vigilant," Homeland Security spokesman Sean Smith said Monday night.

Separately, law enforcement officials said a Colorado man may have been planning with others to detonate backpack bombs on New York City trains in a terrorism plot similar to past attacks on London's and Madrid's mass-transit systems.

The investigation and the earlier warning about mass transit system have already prompted officials around the nation to step up patrols.

Two law enforcement officials speaking on condition of anonymity because they were not authorized to discuss details of the investigation told The Associated Press late Monday that more than a half-dozen individuals were being scrutinized in the alleged plot.

In a statement, the FBI says that "several individuals in the United States, Pakistan and elsewhere" are being investigated.

Investigators say Najibullah Zazi, a 24-year-old Afghanistan-born immigrant who is a shuttle van driver at the Denver airport, played a direct role in the terror plot that unraveled after an overnight 1,600-mile trip from Denver to New York City around the anniversary of the Sept. 11 attacks. He made his first court appearance Monday and remains behind bars.

Zazi and two other defendants have not been charged with any terrorism counts, only the relatively minor offense of lying to the government. But the case could grow to include more serious charges as the investigation proceeds.

Backpacks and cell phones were seized last week from apartments in Queens, where Zazi visited.

Zazi has publicly denied being involved in a terror plot, and defense lawyer Arthur Folsom dismissed as "rumor" any notion that his client played a crucial role.

Publicly, law enforcement officials have repeatedly said they are unaware of a specific time or target for any attacks. Privately, officials speaking on condition of anonymity because they were not authorized to discuss the case said investigators have worried most about the possible use of backpack bombs on New York City trains, similar to attacks carried out in London in 2005 and Madrid in 2004.

Backpack bombs ripped apart four commuter trains and killed 191 people in Madrid on March 11, 2004. On July 7 the next year, bombing attacks in London killed 52 subway and bus commuters.

D.C.'s transit agency increased uniformed patrols around the anniversary of the Sept. 11, 2001, terrorist attacks as it does every year but hasn't elevated security following reports of the plot, and no random bag searches are planned.

"We have no specific information from intelligence sources that would lead us to believe there is a threat to our system at this time, yet MTPD officers remain vigilant,” said Metro Transit Police Chief Michael A. Taborn.